Silver-containing thermographic imaging materials (“direct thermal” materials) are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy. These materials have been known in the art for many years and generally comprise a support having disposed thereon one or more imaging layers comprising (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing composition (usually including a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the energy or image from another material.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. The preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent whereby a black-and-white image of elemental silver is formed.
Problem to be Solved
Thermographic materials are imaged by contacting them with the thermal head of a thermographic recording apparatus such as a thermal printer or thermal facsimile to form a visible image (usually a black-and-white image). Heat generated in the thermal print head can range from 100 to many hundreds of °C. Because the contact between the thermal print head and a given area of the thermographic material is very short (a few milliseconds), the thermographic material never reaches the same temperature as the thermal print head.
It is difficult to generate a “neutral” black-and-white silver image in such materials due to the strong dependence of image tone on silver particle size and shape. Typically, the silver image tends to have a yellowish tint. Thus, a fine balancing of toning agents (“toners”) and other components (such as reducing agents and development accelerators) is necessary to provide a desired “neutral” image tone but even then the image tone can change depending upon imaging conditions (that is, temperature and time). The use of toning agents to adjust image tone in thermally developable materials is a common practice as described in early literature such as U.S. Pat. No. 3,080,254 (Grant, Jr.), U.S. Pat. No. 3,847,612 (Winslow), and U.S. Pat. No. 4,123,282 (Winslow), and in more recent publications of which there are hundreds with U.S. Pat. No. 5,599,647 (Defieuw et al.) and U.S. Pat. No. 6,146,822 (Asanuma et al.) and EP 1,270,255 (Dooms et al.) being representative.
There is a need for better and more predictable control of image tone in thermographic materials that can be imaged under a variety of conditions without the use of silver imaging components.